/Engine/Plugins/MovieScene/MovieRenderPipeline/Source/MovieRenderPipelineCore/Public/MovieRenderOverlappedImage.h
After accumulation is finished, fetch the final image as bytes. In theory we don't need this, because we could just fetch as LinearColor and convert to bytes. But the largest size asked for is 45k x 22.5k, which is 1B pixels. So fetching as LinearColor would create a 16GB intermediary image, so it's worth having an option to fetch straight to FColors.
void FetchFinalPixelDataByte ( TArray64< [FColor](API\Runtime\Core\Math\FColor) > & OutPixelData ) const